The present invention relates in general to speed change mechanisms and in particular to an improved shift motor assembly for selectively moving a shift rod between two gear-engaging positions in a two-speed axle assembly.
Shifting mechanisms are well known in the art for selectively moving a shift fork or similar member linearly between two gear-engaging positions. Such shifting mechanisms usually include a driving device, such as a motor, which is intermittently energized to move the shift fork in a desired direction. Typically, the motor rotates a threaded shaft or screw in either a clockwise or counter-clockwise direction. A drive nut is non-rotatably mounted on the threaded shaft such that rotation of the shaft causes linear movement of the drive nut along the length of the shaft. The direction of such linear movement is determined by the direction of rotation of the threaded shaft. The shift fork is connected to the drive nut so as to be moved linearly therewith.
More recently, some shifting mechanisms have replaced the direct connection between the drive nut and the shift fork with a resilient connection. Such a resilient connection provides a means for biasing or urging the shift fork in the desired linear direction toward the selected gear-engaging position when the drive nut is linearly moved as described above. As a result, the actual movement of the shift fork between the gear-engaging positions may be delayed until the biasing force generated by the resilient connection exceeds any forces tending to resist movement of the shift rod, such as friction caused by torque between the gear-engaging elements. Springs and similar resilient devices have been provided for this purpose. In either type of shifting mechanism, it is desirable to provide some means for limiting the maximum amount of movement of the drive nut along the threaded shaft.